The present invention is related to a superconductive ceramic wire and a method for making same.
It has been known that composite oxides, referred to as superconductive ceramics hereinafter, composed mainly of yttrium-including a rare earth element (denoted hereinafter by R), an alkaline earth metal, copper and oxygen show a superconductivity at a temperature of 77.degree. K. which is higher than the boiling point of nitrogen.
A conventional method for making the above-mentioned superconductive ceramics is as follows.
First, powdery raw materials, that is, an R oxide (R.sub.2 O.sub.3), a carbonate of an alkaline earth metal and copper oxide (CuO), average particle diameter being not larger than 10 .mu.m, are mixed at a molecular proportion of 0.5:2:3, respectively.
Second, the mixed raw materials are heat-treated at a temperature between 850.degree. C. and 950.degree. C. for at least 24 hours in an oxygenous atmosphere to form a superconductive ceramic powder.
In order to obtain a superconductive ceramic wire, the superconductive ceramic powder is sintered to make a sintered block therefrom. The block is then charged in a silver or copper pipe, and a diameter thereof is reduced to be on the order of 5 mm by cold swaging, rolling or dieing. Another conventional method for diametrical reduction is to hot extrude the sintered ceramic block.
A problem of the above-mentioned conventional processing for obtaining a superconductive ceramic wire is that fissures and hollows are apt to be created in the superconductive material during the diametrical reduction because of the poor extensibility of ceramics. As the fissures and hollows spoil superconductivity, namely the critical temperature and the critical electric current density Jc, of the material, the diametrical reduction rate has to be kept lower than a certain level. A disadvantage of the superconductive ceramic wire obtained by a conventional method is that the diameter of the wire is not enough small and the material often has fissures and hollows which spoil the superconductivity. Therefore, electric resistance and critical electric current density which might be obtained in a massive superconductive ceramics is not obtained in the superconductive ceramic wires of the conventional process.